Printed circuit board and method of manufacturing the same

ABSTRACT

The present invention provides a printed circuit board, which includes a dielectric substrate having via holes formed in the thickness direction, and a conductor including a conductive filler is filled in the via holes. The dielectric substrate has patterned wiring layers on both surfaces, and the wiring layers are connected electrically with each other by the conductor. The dielectric substrate is made of a glass cloth or a glass nonwoven fabric impregnated with a thermosetting epoxy resin mixed with fine particles, and the conductive filler in the conductor has an average particle diameter larger than that of the fine particles. Accordingly, the printed circuit board has an improved moisture resistance as a whole and also excellent connection reliability and repair resistance. In addition, the dielectric substrate of the printed circuit board has an improved mechanical strength such as flexural rigidity. The present invention also provides a method of manufacturing such a printed circuit board.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a printed circuit board that issuitable for high density packaging of various high-performanceelectronic apparatuses. The printed circuit board has excellent flexuralrigidity or moisture absorption and repair resistance. This inventionrelates to also a method of manufacturing such a printed circuit board.

[0003] 2. Description of the Related Art

[0004] Recently, various electronic elements composing electronicapparatuses have become smaller and thinner since electronic apparatuseshave become small, thin, light-weight and better in the performance.Printed circuit boards for packaging these electronic elements also havebeen developed to provide a high density package.

[0005] With a rapid progress in packaging techniques, it is desired tosupply circuit boards with a multilayer wiring structure at a low cost.Such a circuit board can be manufactured by mounting bare chips of asemiconductor device such as LSI directly at a high density, and it cancorrespond to a high-speed signal processing circuit. Such a multilayercircuit board is required to have high reliability in electricconnection between wiring patterns of plural layers formed with a minutewiring pitch and also excellent high frequency property, and is requiredto have high reliability in a connection with a semiconductor bare chip.

[0006] For satisfying this requirement, a resin multilayer circuit boardhaving an any-layer IVH structure is proposed in JP-A-6-268345. In thisreference, a copper-plated conductor on a through-hole inner wall, whichmainly has been used for interlayer connection in a conventionalmultilayer circuit board, is replaced by filling of a conductor in ainterstitial via hole (IVH) and at the same time, the IVH can be formedimmediate below an element land or between arbitrary layers, so thatsubstrate size can be decreased and a high density packaging can beprovided. FIGS. 12A-12G show a method of manufacturing such a printedcircuit board. First, as shown in FIG. 12A, mold-releasing films 401made of polyester or the like are laminated on both surfaces of a poroussubstrate 402 such as an aramid epoxy pre-preg prepared by impregnatingan aramid nonwoven fabric with a thermosetting epoxy resin. Next, asshown in FIG. 12B, via holes 403 are formed at predetermined positionsof the porous substrate 402 by laser beam machining. Subsequently asshown in FIG. 12C, a conductive paste 404 is filled in the via holes403. In this step, the porous substrate 402 with the via holes 403 isplaced on a table of a screen printer, and the conductive paste 404 isdirectly printed on the mold-releasing film 401. The mold-releasingfilms 401 on the printing surface function as printing masks and preventcontamination of the surface of the porous substrate 402. Next, themold-releasing films 401 are peeled off from the surfaces of the poroussubstrate 402, and then, metal foils 405 e.g., copper foils, are adheredto the surfaces of the porous substrate 402. By applying heat andpressure in this condition, the porous substrate 402 is compressed andthinned as shown in FIG. 12D. At that time, the conductive paste 404 inthe via holes 403 also is compressed and binder ingredients contained inthe conductive paste is extruded to strengthen bonding among theconductive ingredients and also between the conductive ingredients andmetal foils 405. As a result, the conductive substance in the conductivepaste 404 is condensed and the layers are connected electrically witheach other. Subsequently, the conductive paste 404 and the thermosettingresin composing the porous substrate 402 are cured. The metal foils 405are etched selectively to have a predetermined pattern so that adouble-sided circuit board as shown in FIG. 12E is provided. Further, asshown in FIG. 12F, porous substrates 406 and metal foils 407 are stuckto the surfaces of the double-sided circuit board by using a conductivepaste 408. After applying heat and pressure, as shown in FIG. 12G, themetal foils 407 are etched to have a predetermined pattern, so that amultilayer circuit board is obtained.

[0007] Resin multilayer substrates formed by using such substrates forforming circuits have been used for many electronic apparatuses becausethey have low coefficient of expansion, low dielectric constant, andlight-weight.

[0008] However, a resin multilayer circuit board having theabove-mentioned any-layer IVH structure has a core of an aramid nonwovenfabric, and a dielectric substrate is composed of a homogeneous mixtureof an epoxy resin and aramid nonwoven fabric fibers. Such a dielectricsubstrate having a core of aramid nonwoven fabric has a coefficient ofthermal expansion (CTE) of about 100 ppm/° C. in the thicknessdirection, which is different considerably from CTE (about 17 ppm/° C.)of an interstitial via conductor forming the any-layer IVH structure.

[0009] Since the properties of the electronic apparatus may deteriorateto some degree under a severe use condition with abrupt change intemperature, printed circuit boards with higher reliability have beendesired.

SUMMARY OF THE INVENTION

[0010] In order to solve the above-mentioned problems, a first purposeof the present invention is to provide a printed circuit board and amethod of manufacturing the printed circuit board. The printed circuitboard has an improved moisture resistance as a whole, so that theconnection reliability and repair resistance are improved, and theprinted circuit board comprises a dielectric substrate with improvedmechanical strength e.g., flexural rigidity.

[0011] In addition to the first purpose, a second purpose is to providea printed circuit board with improved adherence between a wiring patternand a dielectric substrate and connection reliability between the wiringpattern and the interstitial via conductor, which is achieved byreducing the coefficient of thermal expansion (CTE) of the entiredielectric substrate.

[0012] For achieving the purposes, a first printed circuit boardaccording to the present invention comprises a dielectric substratehaving via holes formed in the thickness direction in which a conductorcontaining a conductive filler is filled, and wiring layers are formedon both surfaces of the dielectric substrate so as to have apredetermined pattern and connected electrically with each other by theconductor. The printed circuit board is characterized in that thedielectric substrate is made of a glass cloth or a glass nonwoven fabricimpregnated with a thermosetting epoxy resin mixed with fine particles,and the conductive filler contained in the conductor has an averageparticle diameter larger than that of the fine particles.

[0013] A second printed circuit board according to the present inventioncomprises an internal circuit board and external circuit boards providedon both surfaces of the internal circuit board. The internal circuitboard comprises at least two layers of a wiring pattern connected byfirst interstitial via conductors. The external circuit boards comprisea dielectric material of a glass epoxy resin, second interstitial viaconductors compressed into the dielectric substrate to have conductivityand also second wiring patterns arranged on the outermost surfaces ofthe dielectric substrate. The second printed circuit boards arecharacterized in that the first wiring pattern on the surfaces of theinternal circuit board and the second wiring patterns of the externalcircuit boards are connected electrically with each other.

[0014] A first manufacturing method according to the present inventioncomprises steps of:

[0015] preparing a dielectric substrate of a pre-preg formed byimpregnating a glass cloth or a glass nonwoven fabric with athermosetting epoxy resin mixed with fine particles,

[0016] coating both surfaces of the dielectric substrate withmold-releasing films and subsequently forming a via hole,

[0017] filling in the via hole with a conductor containing a conductivefiller having an average particle diameter larger than the averagediameter of the fine particles,

[0018] peeling the mold-releasing films and layering metal foils on thesurfaces of the dielectric substrate,

[0019] compressing the dielectric substrate having the metal foils byapplying heat and pressure in order to adhere the dielectric substrateand the metal foils and to connect electrically the metal foils witheach other, and

[0020] forming the metal foils to have a predetermined pattern.

[0021] A second manufacturing method according to the present inventioncomprises steps of:

[0022] preparing an internal circuit board having at least two layers offirst wiring patterns protruding from surfaces of the internal circuitboard and being connected with each other by first interstitial viaconductors,

[0023] preparing glass epoxy resin dielectric substrates in a pre-pregstage by forming via holes at arbitrary positions and filling aconductive paste to make a second interstitial via conductor,

[0024] arranging the glass epoxy resin dielectric substrates on bothsurfaces of the internal circuit board,

[0025] arranging copper foils on outer surfaces of the respective glassepoxy resin dielectric substrates,

[0026] applying heat and pressure from outside of the two copper foilsthe internal circuit board and the glass epoxy resin dielectricsubstrates in a pre-preg stage so as to force the protruding wiringpatterns of the internal circuit board into the glass epoxy resindielectric substrate in a pre-preg stage and at the same time tocompress the conductive paste provided to the glass epoxy resindielectric substrates for connecting electrically the outermost copperfoils and the first wiring patterns in the internal circuit board, andsubsequently

[0027] etching the copper foils selectively to form second wiringpatterns and so as to configure external circuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIGS. 1A-1F are cross-sectional views to show steps ofmanufacturing a printed circuit board in Example 1 of the presentinvention.

[0029] FIGS. 2A-2D are cross-sectional views to show steps ofmanufacturing a printed circuit board in Example 2 of the presentinvention.

[0030]FIGS. 3A and 3B are cross-sectional views to show steps ofmanufacturing a printed circuit board in Example 3 of the presentinvention.

[0031] FIGS. 4A-4C are distribution charts to show variations inresistance values of the printed circuit board in Example 1 of thepresent invention.

[0032]FIG. 5 is a graph to show a relationship between a thickness of aresin layer of the printed circuit board and initial resistance valuesin Example 1 of the present invention.

[0033]FIG. 6 is a cross sectional view to show a structure of a printedcircuit board in Example 4 of the present invention.

[0034]FIG. 7 is a cross sectional view to show a structure of a printedcircuit board in Example 5 of the present invention.

[0035] FIGS. 8A-8C are cross-sectional views to show steps ofmanufacturing a printed circuit board in Example 6 of the presentinvention.

[0036]FIGS. 9A and 9B are enlarged views to show parts of thecross-sectional views of FIGS. 8A and 8B.

[0037]FIG. 10 is a partially enlarged cross-sectional view to show thatan inorganic filler is mixed with a dielectric substrate shown in FIG.9B.

[0038]FIG. 11 is a graph to show mechanical strength characteristics ofan internal circuit board and of external circuit boards composing theprinted circuit board in Example 6 of the present invention.

[0039] FIGS. 12A-12G are cross-sectional views to show steps ofmanufacturing a conventional printed circuit board.

DETAILED DESCRIPTION OF THE INVENTION

[0040] A first embodiment of the present invention provides a printedcircuit board having excellent connection reliability and repairresistance by improving moisture resistance of the printed circuit boardas a whole, and the printed circuit board comprises a dielectricsubstrate with improved mechanical strength such as flexural rigidityWhen an inorganic filler is mixed with an epoxy resin composing thedielectric substrate, melting viscosity of the epoxy resin is raised.This will control resin flow occurring at an interface between thedielectric substrate and a metal foil at the time of applying heat andpressure, and prevent the epoxy resin from entering the interfacebetween the metal foil and the conductor. As a result, the conductor isprovided with sufficient compression and this leads to stable connectionreliability.

[0041] When the inorganic filler has an average particle diametersmaller than that of the conductive filler, dispersion of the conductivefiller during application of heat and pressure is controlled. As aresult, the conductor is provided with sufficient compression, andstable connection reliability is obtained. Preferably, the averageparticle diameter of the inorganic filler is 10% to 50% of the averageparticle diameter of the conductive filler.

[0042] The conductive filler can be a powder of a metal such as gold,silver, copper, aluminum or the like. When the via hole diameter isabout 50 μm, the conductive filler preferably has an average particlediameter of about 5 μm. Preferably, the conductive filler will have anaverage particle diameter ranging from about 1 μm to about 31 μm for avia hole having a diameter of about 30 μm.

[0043] When the inorganic filler is at least one powder selected fromthose of SiO₂, TiO₂, Al₂O₃, MgO, SiC and AlN, thus obtained printedcircuit board will have further improved mechanical strength includingflexural rigidity.

[0044] When a content of the inorganic filler ranges from 25 vol. % to45 vol. %, the melting viscosity is maintained and sufficientcompression is applied to the conductor. When the content of theinorganic filler is less than 25 vol. %, sufficient melting viscositymay not be obtained and excessive resin flow may occur. When the contentexceeds 45 vol. %, fluidity of the epoxy resin may deteriorate and theconductor cannot be applied with sufficient compression while theadherence is lowered as well. Preferably, the content of the inorganicfiller ranges from 35 vol. % to 40 vol. %.

[0045] When thickness of the dielectric substrate is smaller than adiameter of via holes formed in the thickness direction of thedielectric substrate, tapering at the time of formation of the via holesby using a laser beam machine or the like can be controlled, and thus, athin and light-weight printed circuit board can be provided while therigidity is maintained. Since the conductor will be compressed easily asthe dielectric substrate is thinner (the compression rate is raisedsince the conductor becomes relatively thicker by the thickness of themold-releasing films), the dielectric substrate is preferably 50 μm to100 μm in thickness when each via hole has a diameter ranging from 100μm to 200 μm.

[0046] When the dielectric substrate in a pre-preg stage comprises avoid, the dielectric substrate will be compressed by the void volumeupon application of heat and pressure. As a result, the conductor isapplied with sufficient compression and stable connection resistance isobtained.

[0047] A coefficient of thermal expansion (CTE) in the thicknessdirection of a conductor filled in via holes formed in the thicknessdirection of the dielectric substrate substantially is equal to the CTEin the thickness direction of the dielectric substrate, distortioncaused by a gap in the CTEs can be controlled and stable connectionreliability is obtainable even under a circumstance to cause an abruptchange in temperature. Preferably, the CTE of the conductor in thethickness direction is equal to the CTE of the dielectric substrate inthe thickness direction.

[0048] The printed circuit board can be a multilayer printed circuitboard comprising a laminate of printed circuit boards according to thepresent invention. Accordingly, the present invention can provide amultilayer printed circuit board having reliable via hole connection.

[0049] When a conductor filled in via holes formed in the thicknessdirection of the dielectric substrate is substantially uniform in thethickness in the multilayer printed circuit board of the presentinvention, an excellent impedance property is obtained, and a printedcircuit board suitable for a high frequency circuit board can beprovided.

[0050] In a case where the conductor is a conductive paste, resiningredients in the conductive paste will be discharged from the viaholes when the conductive paste in the via holes is compressed. As aresult, conductive ingredients in the conductive paste are condensed toprovide a reliable via hole connection.

[0051] By using a laser beam machine, via holes having a minute diameterin accordance with a minute wiring pattern can be formed easily at ahigh speed.

[0052] A first manufacturing method in the present invention can providea double-sided printed circuit board that is excellent in the moistureresistance, connection reliability or repair resistance, and the printedcircuit board comprises a dielectric substrate provided with improvedmechanical strength such as flexural rigidity.

[0053] When the dielectric substrate is thinned after application ofheat and pressure, the conductor can be applied with sufficientcompression. For controlling resin flow, it is preferable that thedielectric substrate is thinned by 10% to 15% after application of heatand pressure.

[0054] When the conductor filled in the via holes formed in thethickness direction of the dielectric substrate is thinned after theapplication of heat and pressure, the conductive ingredients in theconductor are condensed due to sufficient compression, and this leads toa reliable via hole connection. Preferably, the conductor is thinned by30% to 40% by the application of heat and pressure.

[0055] When the dielectric substrate after being applied with heat andpressure is substantially uniform in thickness in the center and in theperiphery, the entire dielectric substrate will be applied with uniformcompression at a time of applying heat and pressure, and thus, variationin the connection resistance can be reduced.

[0056] Alternatively, the present invention can provide a rigid, thin,and small multilayer printed circuit board that is provided withexcellent reliability and that can form a minute wiring pattern to allowa high density package of a subminiature electronic element or the like.A method of manufacturing the printed circuit board comprises twoprocesses. In a first process, a dielectric substrate of a pre-preg ismade of either a glass cloth or a glass nonwoven fabric. The glass clothor glass nonwoven fabric is impregnated with a thermosetting epoxy resinand filled with a conductor. Metal foils are adhered to this dielectricsubstrate, and the dielectric substrate is compressed by applying heatand pressure, so that wiring layers of the double-sided printed circuitboard are embedded in resin layers on the surfaces of the dielectricsubstrate and all the conductors become substantially uniform inthickness. In a second process, the metal foils are formed to have apredetermined pattern.

[0057] Alternatively, the present invention can provide a multilayerprinted circuit board with excellent reliability in fewer steps at a lowcost, and the printed circuit board allows high density package ofsubminiature electronic elements or the like. A method to manufacturesuch a printed circuit board comprises preparing either a glass cloth ora glass nonwoven fabric, producing a dielectric substrate of a pre-pregby impregnating the glass cloth or glass nonwoven fabric with athermosetting epoxy resin and filling with a conductor, sticking thedielectric substrate integrally on surfaces of the plural double-sidedprinted circuit boards before compressing by application of heat andpressure. As a result, wiring layers of the double-sided printed circuitboard are embedded in resin layers on the surfaces of the dielectricsubstrate and all the conductors become substantially uniform inthickness.

[0058] A second printed circuit board according to the present inventionis of a complex type with excellent flexural rigidity and moistureresistance, and this printed circuit board is provided by arranging aglass epoxy resin substrate as an external circuit board havingexcellent mechanical strength and moisture resistance.

[0059] It is preferred that an aramid nonwoven fabric impregnated with athermosetting epoxy resin is used for a dielectric substrate of aninternal circuit board. As a result, a first interstitial via conductorhaving a minute diameter can be provided to an arbitrary position on thedielectric substrate so as to provide an any-layer IVH structure. Thisserves to provide a printed circuit board having a high density wiring.

[0060] Preferably, the external circuit board is made of glass fibersimpregnated with an epoxy resin, and the epoxy resin is mixed with aninorganic filler contained in a range from 30 wt % to 70 wt %, and theinorganic filler is at least one powder of SiO₂, TiO₂, Al₂O₃, MgO, SiCand AlN, so that the flexural rigidity is improved further and thus, aprinted circuit board with good compression property can be obtained.

[0061] A second manufacturing method according to the present inventioncan provide a complex printed circuit board having excellent flexuralstrength and moisture resistance.

[0062] Embodiments of the present invention will be explained further byreferring to the drawings.

Example 1

[0063] FIGS. 1A-1F are cross-sectional views to show steps of a methodof manufacturing a double-sided circuit board in Example 1 of thepresent invention.

[0064] First, a dielectric substrate 102 of a glass epoxy pre-preg wasprepared by impregnating a glass cloth with a thermosetting epoxy resincomprising 40 vol. % of SiC fine particles having an average diameter of2 μm. The glass epoxy pre-preg in Example 1 was 130 μm in thickness,since the thickness of the glass cloth was 100 μm and the thickness ofeach surface resin layer was 15 μm. On both surfaces of the dielectricsubstrate 102, mold-releasing films 101 (e.g., polyester) werelaminated. The lamination was carried out at a temperature about 120° C.As a result, resin layers on the surfaces of the dielectric substrate102 to which the mold-releasing films 101 were stuck were meltedslightly. FIG. 5 is a graph to show the dependence of an initialresistance value on the thickness of the resin layer of the materialincluded in a printed circuit board. Since the initial resistance valueis lowered as the resin layer of the material is thinned, the materialin Example 1 was determined to be 130 μm in thickness. Themold-releasing films used here were polyethylene terephthalate (PET) 19μm in thickness. The glass cloth used for the dielectric substrate canbe replaced by a glass nonwoven fabric on which resin layers are formed.

[0065] The fine particles can be an organic filler and/or an inorganicfiller. SiC as the inorganic filler can be replaced by SiO₂, TiO₂,Al₂O₃, MgO or AlN, while the organic filler can include benzoguanamine,polyamide, polyimide, melamine resin, epoxy resin or the like.Alternatively, the organic filler and the inorganic filler can be usedas a mixture.

[0066] As shown in FIG. 1B, via holes 103 were formed at predeterminedpositions on the dielectric substrate 102 by a laser processing. The viaholes formed by the above-mentioned laser beam machine had a diameter ofabout 100 μm. The glass epoxy pre-preg comprises 10 vol. % voids, anddiameter of the voids was 3 μm.

[0067] Next, the via holes 103 were filled with a conductive paste 104as shown in FIG. 1C. A screen printer was used for filling by directlyprinting the conductive paste 104 on the mold-releasing films 101. Atthis time, vacuum adsorption was performed via, for example, a poroussheet such as Japanese paper from a surface opposite to the printedsurface so that resin ingredients contained in the conductive paste 104in the via holes 103 were absorbed and thus, a content of the conductiveingredients was increased and concentration of the filled conductiveingredients was raised. The mold-releasing films 101 functioned asprinting masks and also the films served to prevent contamination of thesurfaces of the dielectric substrate 102. An average particle diameterof the conductive filler in the conductive paste was 5 μm, and this waslarger than a diameter of a void and a diameter of the fine particles.

[0068] As shown in FIG. 1D, the mold-releasing films 101 were peeled offfrom the surfaces of the dielectric substrate 102. Then, the surfaces ofthe dielectric substrate 102 were coated with metal foils 105 such ascopper foils before application of heat and pressure. The heat andpressure were applied by a vacuum press. This process was carried outfor one hour under a condition of a temperature of 200° C., a pressureof 4.9 MPa, and a vacuum degree of 5.33×10³ Pa (40 Torr).

[0069] As a result of applying heat and pressure under the condition,the dielectric substrate 102 was compressed as shown in FIG. 1E. Thedielectric substrate 102 was thinned to 118 μm at the center and 116 μmin the periphery. The difference in the thickness between the center andthe periphery was as small as about 2%, and the compression rate as awhole was about 10%. At this time, the conductive paste 104 in the viaholes 103 were compressed as well, and thus, binder ingredientscontained in the conductive paste were extruded. This strengthensbonding among the conductive ingredients and also between the conductiveingredients and the metal foils 105, and the conductive substance in theconductive paste 104 was condensed. The compression rate of theconductor at this time was (168-117)×100/168=30.4%. Subsequently, thethermosetting resin composing the dielectric substrate 102 was curedtogether with the conductive paste 104 by applying heat and pressure.This process was carried out for one hour under a condition of atemperature of 200° C., a pressure of 4.9 MPa, and a vacuum degree of5.33×10³ Pa (40 Torr).

[0070] Finally, both the wiring layers 106 on the front and backsurfaces of the dielectric substrate 102 were connected electricallywith each other by selectively etching the metal foils 105 in apredetermined pattern as shown in FIG. 1F, so that a double-sidedcircuit board was obtained.

[0071] An average particle diameter of a conductive powder contained inthe conductive paste was 5 μm in Example 1, and an average particlediameter of the inorganic filler was 40% of that of the conductivepowder. The CTE of the conductor in a thickness direction was about 30ppm/° C. while the CTE of the dielectric substrate in the thicknessdirection was about 20 ppm/° C., i.e., CTE of the conductor was largerthan that of the dielectric substrate.

[0072] Since the filler has an average diameter larger than that of thefine particles, the fine particles can prevent dispersion of theconductive filler in the resin layers on the surfaces of the dielectricsubstrate during application of heat and pressure. This helps to applysufficient compression to the conductor, resulting in a stableconnection reliability and excellent moisture absorption. This canprevent peeling of the conductor and the wiring layers, which is causedby abrupt change in temperature at a time of solder dipping orrepairing.

[0073] Furthermore, the epoxy resin composing the dielectric substratewas mixed with 40 vol. % of SiC as an inorganic filler having an averagediameter of 2 μm. FIGS. 4A-4C are distribution diagrams to showvariation in resistance values of printed circuit boards. As shown inFIG. 4A, the resistance values were varied when the filling amount was20 vol. %. On the other hands, FIG. 4B shows that the filler of 30 vol.% caused some variation though the distribution was substantiallyconcentrated. The distribution was concentrated firther when the fillingamount was 40 vol. % as shown in FIG. 4C.

[0074] When the content of the fine particle ranges from 25 vol. % to 45vol. %, the melting viscosity is maintained while the conductor isapplied with sufficient compression, and thus, stable connectionreliability is obtained. When the content is less than 25 vol. %,sufficient melting viscosity is not obtained. As a result, excessiveresin flow occurs in the resin layers on the surfaces of the dielectricsubstrate and the conductive filler contained in the conductor isdispersed, so that pressure applied to the conductor will be dispersedas well. Moreover, the dispersed conductive filler can migrate to causeinsulation failure. When the content exceeds 45 vol. %, sufficient epoxyresin to provide a desired adherence may not be obtained, and thefluidity of the epoxy resin deteriorates to apply insufficientcompression to the conductor. It is more preferable that the content ofthe fine particle ranges from 30 vol. % to 35 vol. %.

[0075] Accordingly, Example 1 can improve mechanical strengths such asflexural rigidity of a dielectric substrate and provide a printedcircuit board having excellent connection reliability and moistureabsorption.

Example 2

[0076] FIGS. 2A-2D are cross-sectional views to indicate steps of amethod of manufacturing a double-sided circuit board in Example 2 of thepresent invention.

[0077] First, a core substrate 201 was prepared in the same process asindicated in FIG. 1F of Example 1.

[0078] Next, as shown in FIG. 2B, dielectric substrates 202 shown inFIG. 1C were layered on the both surfaces of the core substrate 201, andmetal foils 203 were layered firther before application of heat andpressure. The heat and pressure were applied by vacuum heat press. Thisprocess was carried out for one hour under a condition of a temperatureof 200° C., a pressure of 4.9 MPa, and a vacuum degree of 5.33×10₃ Pa(40 Torr).

[0079] The dielectric substrates 202 were compressed and thinned by theapplication of heat and pressure, and the wiring layers 204 wereembedded in the dielectric substrates 202. The conductive paste 205 alsowas compressed, and at that time, binder ingredients contained in theconductive paste were extruded. This provided strong bonding among theconductive ingredients and also between the conductive ingredients andthe metal foils 203, and thus, the conductive substance in theconductive paste 205 was condensed. Subsequently, the conductive paste205 and the thermosetting resin composing the dielectric substrates 202were cured.

[0080] Next, as shown in FIG. 2C, the metal foils 203 were etchedselectively to have a predetermined pattern, so that the wiring layers204 and 206 were connected electrically to provide a completedfour-layered circuit board.

[0081] In the last step shown in FIG. 2D, dielectric substrates 207 werelayered on both surfaces of the four-layered circuit board. The wiringlayer 206 was connected electrically with the wiring layer 208 throughsteps as shown in FIGS. 2B and 2C, so that a six-layered circuit boardwas obtained.

[0082] All layers of the six-layered circuit board according to Example2 were composed of glass epoxy, and laminated integrally. Therefore,compact wiring patterns to allow high density packaging of elements suchas a subminiature electronic element can be formed, and the printedcircuit board has excellent rigidity, moisture absorption, and repairproperty.

Example 3

[0083]FIGS. 3A and 3B are cross-sectional views to show steps in amethod of manufacturing a double-sided printed circuit board accordingto Example 3 of the present invention.

[0084] As shown in FIG. 3A, three core substrates 301 provided by thestep shown in FIG. 1F, and two dielectric substrates 302 provided by thestep shown in FIG. 1C were prepared. The respective dielectricsubstrates 302 were inserted between the core substrates 301 to form alaminate before application of heat and pressure. The heat and pressurewere applied by a vacuum heat press. This process was carried out forone hour under a condition of a temperature of 200° C., a pressure of4.9 MPa, and a vacuum degree of 5.33×10³ Pa (40 Torr).

[0085] As a result of application of heat and pressure, the dielectricsubstrates 302 were compressed and thinned as shown in FIG. 3B, and thewiring layers 303 were embedded in the dielectric substrates 302. Theconductive paste 304 also was compressed at this time, and the binderingredients contained in the conductive paste were extruded tostrengthen bonding among the conductive ingredients and also between theconductive ingredients and the wiring layers 303, and thus, theconductive substance in the conductive paste 304 was condensed. Later,the conductive paste 304 and the thermosetting resin composing thedielectric substrates 302 were cured by the application of heat andpressure, and the wiring layers 303 were connected electrically, so thata six-layered circuit board was obtained.

[0086] All layers of a six-layered circuit board according to Example 3are composed of glass epoxy, and laminated integrally. Therefore,compact wiring patterns to allow high density packaging of elements suchas a subminiature electronic element can be formed with fewer steps at alow cost, and the printed circuit board has excellent rigidity, moistureabsorption, and repair property.

Example 4

[0087]FIG. 6 shows a printed circuit board in Example 4 of the presentinvention. An external circuit board 6 comprising dielectric substrates5 of a glass epoxy resin was manufactured in accordance with Example 1.An internal circuit board 4 in this example has a three-layeredany-layer IVH structure having dielectric substrates 3 of an aramidepoxy resin in which a first interstitial via conductor la and a firstwiring pattern 2 a were formed. On both surfaces of the internal circuitboard 4, the external circuit boards 6 comprising dielectric substrates5 of a glass epoxy resin having a second wiring pattern 2 on outermostlayers connected with the first wiring pattern 2 a of the internalcircuit board 4 through a second interstitial via conductor 1 b werearranged. A conductive paste filled in the via holes was compressed tobe the second interstitial via conductor 1 b when the external circuitboards 6 were laminated on the internal circuit board 4. The thus formedsecond interstitial via conductor 1 b had sufficient conductivity.

Example 5

[0088]FIG. 7 shows a printed circuit board in Example 5 of the presentinvention. The printed circuit board is same as that of Example 4 exceptthat the internal circuit board 4 is a double-sided circuit boardcomprising a dielectric substrate 3 of an aramid epoxy resin. Theprinted circuit board had a structure similar to that of Example 1except for the above-identified matter.

Example 6

[0089] In the following, a method of manufacturing a printed circuitboard in Example 6 will be described by referring to FIGS. 8A-8C wheresome of the elements common to those of FIG. 6.

[0090]FIG. 8A shows a structure of a printed circuit board according toExample 6. An internal circuit board 4 has an any-layer IVH structure inwhich first wiring patterns 2 a are connected with each other by a firstinterstitial via conductor 1 a. On both surfaces of the internal circuitboard 4, dielectric substrates 5 comprising a glass epoxy resin in apre-preg stage are arranged, and the dielectric substrates 5 are filledwith a conductive paste 7 by printing or the like. The conductive paste7 comprises a conductive powder such as silver or copper as a mainingredient for forming a second interstitial via conductor 1 b. Copperfoils 8 are arranged further on the surfaces.

[0091] Subsequently, as shown in FIG. 8B, the entire body is compressedand laminated by applying heat and pressure from both sides by using apress such as a mold. As a result, the first wiring patterns 2 a formedin the outermost layers of the internal circuit board 4 are forced intothe surface layers of the glass epoxy resin dielectric substrates 5 thathave been in a pre-preg stage before the compression, and at the sametime, the conductive paste 7 provided in the glass epoxy resindielectric substrates 5 is compressed to have conductivity so that theconductive paste 7 becomes the second interstitial via conductor 1 b toconnect the first wiring patterns 2 a and the copper foils 8electrically. At this time, the copper foils 8 are adhered stronglythrough the epoxy resin layers flowing on the surfaces of the dielectricsubstrates 5.

[0092] Next, as shown in FIG. 8C, the copper foils 8 as outermost layersadhered to the surfaces are patterned selectively by an ordinaryphotolithography in order to form second wiring patterns 2 b, so thatthe first wiring patterns 2 a and the second wiring patterns 2 b areconnected to each other through the first interstitial via conductor 1 aand the second interstitial via conductor 1 b in order to provide acomplex printed circuit board having a multilayer structure.

[0093]FIG. 9A is a magnified view of a part of the internal circuitboard 4 concerning a manufacturing method shown in FIG. 8A, in which adielectric substrate 5 and a copper foil 8 are laminated. The dielectricsubstrate 5 shown in FIG. 9A comprises a glass epoxy resin in a pre-pregstage, in which glass fibers 5 a aggregate in the inner layer of thedielectric substrate 5 and epoxy resin 5 b is retained on the surface ofthe dielectric substrate 5.

[0094] Therefore, the dielectric substrate 5, the internal circuit board4 and the copper foil 8 are arranged as shown in FIG. 9A before applyingheat and pressure from both sides, so that the wiring pattern 2 a of theinternal circuit board 4 is forced into the epoxy resin 5 b retained onthe surface of the dielectric substrate 5.

[0095] At the same time, the conductive paste 7 filled in the via holesin the dielectric substrate 5 is compressed to decrease the initialthickness of t₁ to t₂, so that a second interstitial via conductor 1 bhaving a low resistance is formed to allow electrical connection betweenthe copper foil 8 and the first wiring pattern 2 a of the internalcircuit board 4.

[0096]FIG. 10 shows an external circuit board 10 in which a dielectricsubstrate similar to that of FIGS. 9A and 9B comprises a glass epoxyresin mixed with 30 wt % to 70 wt % of at least one inorganic filler 9comprising powders of SiO₂, TiO₂, Al₂O₃, MgO, SiC, AlN or the like.Addition of the inorganic filler 9 raises the viscosity of the epoxyresin, and this can prevent the dielectric epoxy resin from entering theinterface between the first wiring pattern 2 a or the copper foil 8 andthe conductive paste. As a result, the electric connection reliabilityis improved, and the mechanical strength of the printed circuit boardcan be improved. The conductive paste is applied with heat and pressureto be the second wiring pattern 1 b.

[0097] Since the CTE direction of the dielectric substrate in thethickness can be decreased to a range from 20 ppm to 30 ppm by mixingthe inorganic filler, the value approaches the CTE of an interstitialvia conductor (about 17 ppm), and this improves connection reliabilitybetween the wiring pattern and the interstitial via conductor.Furthermore, a coefficient of thermal expansion in the face direction ofthe printed circuit board can be decreased as well, and thus, theconnection reliability can be improved at a time of packaging asemiconductor chip.

[0098]FIG. 11 is a graph to show a measurement result concerningflexural rigidities of an internal circuit board 4, an external circuitboard 6 and an external circuit board 10 comprising an inorganic fillerof a printed circuit board according to the present invention. Theexternal circuit board 6 made of a glass epoxy resin has a flexuralrigidity higher than that of the internal circuit board 4 comprising anaramid epoxy resin. The external circuit board 10 additionallycomprising an inorganic filler has an even higher flexural rigidity.

[0099] A printed circuit board according to the present inventioncomprises an external circuit board 6 or 10 comprising a glass epoxyresin arranged on both sides of an internal circuit board 4 comprisingan aramid epoxy resin. Such a printed circuit board can have excellentpackaging reliability for respective electronic elements, and alsorepair property and moisture resistance for the elements.

[0100] The mechanical strength cannot be expected to improve furtherwhen the content of the inorganic filler is less than 30 wt %. On theother hand, when the content exceeds 70 wt %, the epoxy resin will haveinferior fluidity, and impregnating ability with respect to the glassfibers deteriorates. As a result, it will be difficult to obtain ahomogeneous glass epoxy resin dielectric substrate.

[0101] The invention may be embodied in other forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not limiting. The scope of the invention is indicatedby the appended claims rather than by the foregoing description, allchanges that come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. A printed circuit board comprising a dielectricsubstrate having a via hole formed in a thickness direction of thedielectric substrate and filled with a conductor containing a conductivefiller, the dielectric substrate having wiring layers on both surfaces,and the wiring layers being formed to have a predetermined pattern andconnected electrically with each other by the conductor, wherein thedielectric substrate comprises a glass cloth or a glass nonwoven fabricimpregnated with a thermosetting epoxy resin mixed with fine particles,and the conductive filler in the conductor has an average particlediameter larger than an average diameter of the fine particles.
 2. Theprinted circuit board according to claim 1 , wherein the fine particlesare at least one inorganic filler selected from powders of SiO₂, TiO₂,Al₂O₃, MgO, SiC and AlN.
 3. The printed circuit board according to claim1 , wherein the conductor filled in the via hole formed in the thicknessdirection of the dielectric substrate has a coefficient of thermalexpansion larger than a coefficient of thermal expansion of thedielectric substrate in the thickness direction.
 4. The printed circuitboard according to claim 1 , wherein a content of the fine particlesranges from 25 vol. % to 45 vol. %.
 5. The printed circuit boardaccording to claim 1 , wherein the thickness of the dielectric substrateis smaller than a diameter of the via hole formed in the thicknessdirection of the dielectric substrate.
 6. The printed circuit boardaccording to claim 1 , wherein the dielectric substrate in a pre-pregstage comprises a void.
 7. The printed circuit board according to claim1 , comprising a single printed circuit board.
 8. The printed circuitboard according to claim 1 , comprising a multilayer printed circuitboard of a laminate of a plurality of the printed circuit boards.
 9. Theprinted circuit board according to claim 8 , wherein the conductorfilled in the via hole formed in the thickness direction of thedielectric substrate of the multilayer printed circuit board has asubstantially uniform thickness.
 10. A printed circuit board comprising:an internal circuit board having a first wiring pattern of at least twolayers connected by a first interstitial via conductor, and externalcircuit boards that are provided on both surfaces of the internalcircuit board and comprise a second interstitial via conductor providedwith conductivity by being compressed in glass epoxy resin as andielectric substrate, and second wiring patterns arranged on outermostsurfaces of the dielectric substrate, wherein the second interstitialvia conductor connects electrically the first wiring patterns on thesurfaces of the internal circuit board and the second wiring patterns ofthe external circuit boards.
 11. The printed circuit board according toclaim 10 , wherein the internal circuit board is a multilayer printedcircuit board having an any-layer interstitial via hole structure, wherethe dielectric substrate is an aramid non-woven fabric impregnated witha thermosetting epoxy resin, and the first wiring patterns are connectedelectrically by the first interstitial via conductor provided in thedielectric substrate.
 12. The printed circuit board according to claim10 , wherein plural external circuit boards are formed on at least onesurface of the internal circuit board.
 13. The printed circuit boardaccording to claim 10 , wherein the epoxy resin composing the externalcircuit board is mixed with an inorganic filler.
 14. The printed circuitboard according to claim 13 , wherein the inorganic filler is at leastone powder selected from SiO₂, TiO₂, Al₂O₃, MgO, SiC and AlN.
 15. Theprinted circuit board according to claim 13 , wherein a content of theinorganic filler ranges from 30 wt % to 70 wt %.
 16. A method ofmanufacturing a printed circuit board, comprising: preparing adielectric substrate of a pre-preg formed by impregnating a glass clothor a glass nonwoven fabric with a thermosetting epoxy resin mixed withfine particles, coating both surfaces of the dielectric substrate withmold-releasing films and subsequently forming a via hole, filling in thevia hole with a conductor containing a conductive filler having anaverage particle diameter larger than an average diameter of the fineparticles, peeling the mold-releasing films and layering metal foils onthe surfaces of the dielectric substrate, compressing the dielectricsubstrate having the metal foils by applying heat and pressure in orderto adhere the dielectric substrate and the metal foils and to connectelectrically the metal foils with each other, and forming the metalfoils to have a predetermined pattern.
 17. The method of manufacturing aprinted circuit board according to claim 16 , wherein the resin layerson the surfaces of the pre-preg have a thickness ranging from 5 μm to 25μm.
 18. The method of manufacturing a printed circuit board according toclaim 16 , wherein the dielectric substrate in a pre-preg stagecomprises a void.
 19. The method of manufacturing a printed circuitboard according to claim 16 , wherein the void has a diameter smallerthan a diameter of the conductive filler in the conductor.
 20. Themethod of manufacturing a printed circuit board according to claim 16 ,wherein the dielectric material is thinned by applying heat andpressure.
 21. The method of manufacturing a printed circuit boardaccording to claim 16 , wherein the conductor filled in the via holeformed in the thickness direction of the dielectric substrate is thinnedby applying heat and pressure.
 22. The method of manufacturing a printedcircuit board according to claim 16 , wherein the dielectric substratehas a substantially uniform thickness in the center and in the peripheryafter application of heat and pressure.
 23. The method of manufacturinga printed circuit board according to claim, 16, further comprising:filling a conductor in a dielectric substrate composed of a pre-preg ofa glass cloth or a glass nonwoven fabric impregnated with athermosetting epoxy resin mixed with fine particles, layering thedielectric substrate and metal foils onto both surfaces of a printedcircuit board prepared in accordance with claim 16 , compressing theprinted circuit board by applying heat and pressure so as to embedwiring layers of the printed circuit board in the resin layers on thesurfaces of the dielectric substrate, and forming the metal foils tohave a predetermined pattern.
 24. The method of manufacturing a printedcircuit board according to claim 16 , comprising: forming a plurality ofdouble-sided printed circuit boards according to claim 16 , filling aconductor in a dielectric substrate composed of a pre-preg of a glasscloth or a glass nonwoven fabric impregnated with a thermosetting epoxyresin mixed with fine particles, layering the dielectric substratebetween the double-sided printed circuit boards, compressing the printedcircuit board by applying heat and pressure so as to embed the wiringlayers of the double-sided printed circuit board in the resin layers onthe surfaces of the dielectric substrate, with all the conductors havingsubstantially same thickness.
 25. A method of manufacturing a printedcircuit board, comprising: preparing an internal circuit board having atleast two layers of first wiring patterns connected with each other byfirst interstitial via conductors, preparing glass epoxy resindielectric substrates in a pre-preg stage by forming via holes in whicha conductive paste for forming second interstitial via conductors isfilled, arranging the glass epoxy resin dielectric substrates on bothsurfaces of the internal circuit board, arranging copper foils on outersurfaces of the respective glass epoxy resin dielectric substrates,applying heat and pressure from outside of the two copper foils to theinternal circuit board and the glass epoxy resin dielectric substratesin a pre-preg stage so as to force the wiring patterns protruding fromthe surfaces of the internal circuit board into the glass epoxy resindielectric substrate in a pre-preg stage and at the same time tocompress the conductive paste provided in the glass epoxy resindielectric substrates for connecting electrically the outermost copperfoils and the first wiring patterns in the internal circuit board, andsubsequently etching the copper foils selectively to form second wiringpatterns and so as to configure external circuit boards.
 26. The methodof manufacturing a printed circuit board according to claim 25 , whereinat least one inorganic filler selected from powders of SiO₂, TiO₂,Al₂O₃, MgO, SiC and AlN is added to the glass epoxy resin dielectricsubstrates in a pre-preg stage and a content of the inorganic fillerranges from 30 wt % to 70 wt % to the entire epoxy resin.